The method of electrospray ionization in ambient gas at atmospheric pressure suffers from the fact that not all the spray droplets created evaporate completely. An inlet capillary with an inside diameter of around 400 to 600 micrometers should really introduce only ambient gas with analyte ions into the mass spectrometer. It happens time and time again, however, that spray droplets enter the vacuum of the mass spectrometer through the inlet capillary and generate interfering charges somewhere in the mass spectrometer.
There is a long list of patents intended to prevent these charges developing. According to these patents, the inlet capillary must not aim directly at the through-hole for ions in the gas skimmer between the first and second pump stages (U.S. Pat. No. Re. 35,413, I. C. Mylcreest, M. E. Hail); the inlet capillary can be offset parallel to the axis of the mass spectrometer (U.S. Pat. No. 5,481,107, T. Yasuaki et al.); the inlet capillary can form an angle with the axis of the mass spectrometer (U.S. Pat. No. 5,818,041, A. Mordehai, S. E. Buttrill); the path of the ions into the vacuum system can be multiply kinked (U.S. Pat. No. 5,756,994, S. Bajic); or the spray direction can be chosen to be orthogonal to the inlet capillary so that the inertia of the droplets causes them to fly past the inlet capillary (U.S. Pat. No. 5,750,988, J. A. Apffel et al.).
The droplets cause interferences inside the mass spectrometer because their impact on instrument components in the vacuum leads to charged surfaces, and considerable efforts are made to prevent them gaining access to the mass spectrometer.
Conversely, for a decade and more, work has occasionally been carried out which generates highly charged droplets by electrospray in the vacuum of a mass spectrometer, accelerates them and fires them onto an impact plate (target), where they burst and release macromolecules in charged form. The molecules can be contained in the sprayed liquid (see for example: “Impact desolvation of electrosprayed microdroplets—a new ionization method for mass spectrometry of large biomolecules”, S. A. Aksyonov and P. Williams, Rapid Communications in Mass Spectrometry, 2001; 15; 2001-2006); they can also be adsorbed on the impact plate (“Formation of multiply charged ions from large molecules using massive-cluster impact”, J. F. Mahoney et al., Rapid Comm. in Mass Spectrom. 1994, 8, 403-406).
If the analyte molecules are contained in the microdroplets, the authors term this IDEM (Impact Desolvation of Electrosprayed Microdroplets); if the analyte molecules are adsorbed on a surface then the method is known as MCI (Massive Cluster Impact). The microdroplets produced in a vacuum have also been used directly to clean the surfaces of adsorbed impurities (“Surface cleaning using energetic microcluster beams”, J. F. Mahoney et al., Solid State Technology, 1998, 41, 149). Electrospray ionization in a vacuum is difficult, however, and considerable effort is required to achieve a continuously operating jet of droplets. Glycerol is often used as the spray liquid.
This type of ionization and for cleaning surfaces in a vacuum using microdroplets, is made possible by electrical acceleration of the droplets, which produces a sufficiently high kinetic energy. The input of kinetic energy is necessary because evaporation causes the microdroplets in the vacuum to cool to such an extent that they can no longer evaporate by themselves. They can evaporate only when the kinetic energy is converted into thermal energy. Insufficient kinetic energy for a complete evaporation also explains the charge phenomena which are observed with electrospray ionization in the mass spectrometer if no measures are put in place to prevent microdroplets entering.
The familiar ionization of dissolved analyte substances by electrospray in ambient gas at atmospheric pressure for their mass spectrometric analysis assumes, on the other hand, that the evaporation of the spray droplets at atmospheric pressure in the ambient gas is as complete as possible. The gas is strongly heated (to a few hundred degrees Celsius) as a rule. However, evaporation can also occur in the ambient gas even without high kinetic energy and without heating the ambient gas: Highly charged microdroplets spatter even on relatively gentle contact with a surface without a high kinetic energy having to be present, as is familiar from very recent experiments with microdroplets at atmospheric pressure (“Mass Spectrometry Sampling Under Ambient Conditions with Desorption Electrospray Ionization”, Z. Takats et al. Science, 2004; 306, 471-473). The microsplashes then evaporate completely, probably by absorbing thermal energy from the unheated ambient gas. This effect can be used to generate ions from analyte molecules which are adsorbed on these surfaces.
Electrospray is predominantly used in two different embodiments: normal electrospray and nanoelectrospray.
Normal electrospray operates using spray capillaries with inside diameters of between 200 and 300 micrometers and spray voltages of between four and five kilovolts. The strong electric field at the tip of the spray capillary polarizes the surface of the spray liquid, usually water mixed with organic solvents, forms the liquid surface to a so-called “Taylor cone”, and pulls a jet out of its tip which resolves into fine microdroplets. Nowadays, the process is usually assisted by a spray gas, which is sharply blown in the coaxial direction. The droplets have diameters of around one micrometer and in some cases a little more. They evaporate predominantly in hot curtain gas, usually nitrogen, fed in a direction counter to that of the droplets, partly by bursting because of the Coulomb charge pressure, and partly by the evaporation of neutral particles and light ion clusters. Dissolved analyte molecules remain behind in charged form (U.S. Pat. No. 4,531,056, M. J. Labowsky, J. B. Fenn, M. Yamashita). The analyte ions can be introduced via an inlet capillary into the vacuum system of a mass spectrometer, even against a potential difference, and can be analyzed there (U.S. Pat. No. 4,542,293, J. B. Fenn et al.). Professor John B. Fenn was awarded the 2002 Nobel Prize in Chemistry for developing this method in the 1980s.
Normal electrospray has stable and unstable spray states. A stable spray state produces droplets of almost identical diameter; slightly unstable spray states, whose presence is apparent from an oscillation of the spray current, supply droplets of various diameters. The slightly unstable mode often provides a larger number of useable analyte ions, but overall, with normal electrospray, only a very small fraction of the analyte ions are ever collected through the suction cone in front of the inlet capillary and transferred to the mass spectrometer.
With nanoelectrospray, the spray capillary is thin at the tip, creating an aperture with a diameter of only approx. four micrometers (U.S. Pat. No. 5,504,329, M. Mann et al.). This electrospray ionization is operated with voltages below one kilovolt and provides microdroplets with diameters of only 100 to 200 nanometers, i.e. only around a thousandth of the volume of normal spray droplets. It is much easier to completely vaporize these microdroplets in hot nitrogen. The spray jet can be sprayed directly into the inlet capillary. Occasionally, this method also develops charges in the mass spectrometer, caused by microdroplets which are not completely vaporized. This nanoelectrospray ionization provides the most efficient utilization of the analyte molecules; no other method of ionization has been elucidated which has such a high yield of analyte molecule ions. Analyte ions are lost only in the inlet capillary.
Both methods of electrospray ionization operate according to the existing prior art with a differential pump system, in which a so-called “skimmer” is employed between the first and second pump stages. The skimmer deflects most of the gas flow, which is blown out of the inlet capillary into the first stage, in such a way that it does not decelerate and break the gas jet of the inlet capillary by direct reflection, causing the gas to back up. Only the central part of the gas jet enters the second stage of the pump system through the aperture of the skimmer; this part contains some of the ions but not sufficient to be satisfactory.
Some laboratories are now working on means of replacing the skimmer and increasing the yield of transmitted ions. The ion funnel is a successful arrangement of this type. The ion funnel operates with RF voltage in order to keep the analyte ions away from the walls of the funnel, and with DC voltage to guide them to the narrow funnel aperture (U.S. Pat. No. 6,107,628; R. D. Smith and S. A. Shaffer, also U.S. Pat. No. 5,572,035 A; J. Franzen). From there they are guided to the mass analyzer.
The ion funnel has a relatively large exit aperture, as otherwise, light ions are reflected in the ion funnel. But this allows a damagingly large amount of gas from the gas jet emerging from the inlet capillary to penetrate into the subsequent pump stages, and—in the case of existing arrangements where the mass analyzer lies precisely in the axis of the inlet capillary—the gas can continue as far as the mass analyzer. With some mass spectrometers, such as ion cyclotron resonance mass spectrometers, which only operate well with the best possible ultra-high vacuum, this gas jet is extremely damaging. The interior of the ion funnel can therefore be equipped with a “jet disturber” (U.S. Pat. No. 6,583,408 B2, R. D. Smith et al.), an impact plate which interrupts the passage of the jet into the mass analyzer. The deflected analyte ions are collected again by the surrounding ion funnel and guided to the mass analyzer.